Typically, a solid oxide fuel cell (SOFC) employs an electrolyte of ion-conductive solid oxide such as stabilized zirconia. The electrolyte is interposed between an anode and a cathode to form an electrolyte electrode assembly (MEA). The electrolyte electrode assembly is interposed between separators (bipolar plates). In use, normally, predetermined numbers of the electrolyte electrode assemblies and the separators are stacked together to form a fuel cell stack.
As the fuel gas supplied to the solid oxide fuel cell, normally, a hydrogen gas, Co, or methane generated from hydrocarbon raw material by a reformer is used. In general, in the reformer, a reformed raw material gas is obtained from hydrocarbon raw material of a fossil fuel or the like, such as methane or LNG, and the reformed raw material gas undergoes steam reforming, partial oxidation reforming, or autothermal reforming to produce a reformed gas (fuel gas). The operating temperature of the solid oxide fuel cell is high, at the temperature of several hundred ° C. (e.g., about 600° C. to 800° C.). Therefore, radiation of the heat to the outside becomes large, and heat efficiency is lowered. Further, heat insulating capacity (volume) for containing the solid oxide fuel cell in a thermally insulated state becomes significantly large, and the cost for heat insulation becomes high.
In an attempt to address the problem, for example, Japanese Laid-Open Patent Publication No. 2001-068135 discloses a fuel cell reforming system as shown in FIG. 9. The fuel cell reforming system includes a reformer 1a, a high temperature shift reactor 2a, a low temperature shift reactor 3a, and a selective oxidizer 4a, and these components are covered by separate pieces of inner heat insulating material 5a. The respective pieces of inner heat insulating material 5a are covered by outer heat insulating material 6a, and a heat collection heat exchanger 7a for collecting the waste heat of the respective reactors (1a, 2a, 3a, 4a) is provided inside the outer heat insulating material 6a. 
Further, as shown in FIG. 10, a fuel cell power generation system disclosed in Japanese Laid-Open Patent Publication No. 2004-087362 includes a fuel cell 1b, a reformer 2b for reforming a fuel gas supplied to the fuel cell 1b, a combustor for burning an exhaust fuel gas from the fuel cell 1b, and a heat collection heat exchanger 4b for performing heat exchange between the combustion exhaust gas from the combustor 3b and the air to be supplied to the fuel cell 1b. 
The fuel cell 1b, the reformer 2b, the combustor 3b, and the heat collection heat exchanger 4b are disposed in a casing 5b, and the casing 5b is made of heat insulating material. The casing 5b includes a casing body 6b and a heat insulating layer 7b covering the casing body 6b. The casing body 6b is made of high temperature heat insulating material, and the heat insulating layer 7b is made of heat insulating material having small heat conductance in comparison with the high temperature heat insulating material of the casing body 6b. 
Further, as shown in FIG. 11, heat insulating container structure disclosed in Japanese Laid-Open Patent Publication No. 2005-194123 has dual structure where a container is includes an inner heat insulating layer 2c and an outer heat insulating layer 3c. A channel 4c as a passage of a combustion gas produced by burning a fuel using the combustion air is formed inside the inner heat insulating layer 2c, and a supply channel 5c for the combustion air is formed between the inner heat insulating layer 2c and the outer heat insulating layer 3c. A reformer 6c and a combustor 7c are provided in the channel 4c. The combustion air which flows into the supply channel 5c is heated by the combustion gas in the channel 4c, and then supplied to the area inside the inner heat insulating layer 2c. 
However, in Japanese Laid-Open Patent Publication No. 2001-068135, the reformer 1a, the high temperature shift reactor 2a, the low temperature shift reactor 3a, and the selective oxidizer 4a are covered by the separate pieces of the inner heat insulating material 5a. Therefore, the fuel cell reforming system has complicated structure, and the production cost becomes high. Further, since the inner heat insulating material 5a and the outer heat insulating material 6a do not have different capabilities depending on the functions of the units to be covered, improvement in the overall heat efficiency in the reforming system cannot be achieved.
Further, in Japanese Laid-Open Patent Publication No. 2004-087362, there is large space between the fuel cell 1b as a high temperature apparatus, the reformer 2b, the heat collection heat exchanger 4b, and the casing body 6b. Therefore, in the casing body 6b, convection flows are generated easily, and thus, heat radiation may be facilitated undesirably. Further, though the relationship of the heat conductance between the casing body 6b and the heat insulating layer 7b is disclosed, in particular, it is not possible to suppress heat radiation from the high temperature apparatus.
Further, in Japanese Laid-Open Patent Publication No. 2005-194123, the inner heat insulating layer 2c and the outer heat insulating layer 3c of the container 1c simply have the heat insulating capability. Therefore, in particular, it is not possible to suppress radiation of heat from the reformer 6c and the combustor 7c, and reduction in the heat insulating capacity cannot be reduced.
The above and other objects, features and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which preferred embodiments of the present invention are shown by way of illustrative example.